Scala subtype check

console

scala> val fs = Field("string")
fs: Field[String] = Field(string)

scala> val fi = Field(1)
fi: Field[Int] = Field(1)

scala> val O = Update[Object]
O: Update[Object] = sparken.Update$$anon$1@4b41956d

// Runtime check
scala> O.update(fs, 1)
res0: sparken.Field[_] = Field(1) // Updated

scala> O.update(fi, "foobar")
res2: sparken.Field[_] = Field(1) // Not updated

// Compile time check
scala> O.updateSubtype(fs, 1)
res3: sparken.Field[Int] = Field(1) // Updated

scala> O.updateSubtype(fi, "foobar") // Compile time error
<console>:13: error: Cannot prove that Int <:< Object.
              O.updateSubtype(fi, "foobar")
                             ^

sub.scala

import scala.reflect.runtime.universe._

case class Field[A](value: A)

trait Update[A] {

  def evA: TypeTag[A]

  def update[B: TypeTag, C](f: Field[B], c: C): Field[_] = f match {
    case _: Field[_] if typeTag[B].tpe <:< evA.tpe => Field(c)
    case _ => f
  }

  // Without reflection, compile time check
  def updateSubtype[B, C](f: Field[B], c: C)(implicit ev: B <:< A) =
    Field(c)
}

object Update {
  implicit def apply[A: TypeTag]: Update[A] =
    new Update[A] { val evA = typeTag[A] }
}